Nineteen people have been killed and more than 50 injured in a suspected terror attack at Manchester Arena.
The blast happened at 22:35 BST on Monday following a pop concert by the US singer Ariana Grande.
PM Theresa May said her thoughts were with those affected by “what is being treated by the police as an appalling terrorist attack”.
Paramedics at the scene told the BBC they had treated some of the wounded for “shrapnel-like injuries”.
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North West Ambulance Service said it had taken 59 casualties from the explosion to hospitals and treated a number of walking wounded.
Greater Manchester Police has established an emergency telephone number in response to the attack. It is: 0161 856 9400.
The prime minister has suspended Conservative Party general election campaigning and will chair a meeting of the government’s emergency Cobra committee later, in response to the attack.
Mrs May said: “We are working to establish the full details” of what happened in Manchester.
“All our thoughts are with the victims and the families of those who have been affected,” Mrs May said.
BBC Home Affairs Correspondent Daniel Sandford said senior counter-terrorism officers were assembling in London and liaising with the Home Office.
Unconfirmed reports from two unnamed US officials suggested the attack was carried out by a suicide bomber.
Media captionDashcam appears to capture the moment of the blast
Labour leader Jeremy Corbyn tweeted: “Terrible incident in Manchester. My thoughts are with all those affected and our brilliant emergency services.”
Greater Manchester metro mayor Andy Burnham said: “My heart goes out to families who have lost loved ones, my admiration to our brave emergency services. A terrible night for our great city.”
British Transport Police said the explosion was in the arena’s foyer, which connects with Victoria train and tram station, a major hub on the northern edge of the city centre.
Shortly after the blast the station was closed and all trains cancelled.
Greater Manchester Police carried out a precautionary controlled explosion in the Cathedral Garden area of the city at about 01:32. The force confirmed it was not a suspicious item.
Media captionManchester Arena incident: Eyewitness describes ‘explosion’
The arena explosion occurred shortly after Ariana Grande left the stage at the arena, the city’s largest indoor venue with a concert capacity of around 18,000.
Grande – a 23-year-old American TV teen actress-turned-pop star – has a strong following among teenage girls and children.
It is believed the blast, which shook the building, centred on the area near the box office where concert merchandise is often sold.
The pop star tweeted: “broken. from the bottom of my heart, i am so so sorry. i don’t have words.”
Pop star’s sorrow at concert deaths
Media captionFootage shows a string of emergency services vehicles rushing to the scene
In the aftermath of the explosion witnesses spoke about the fear and confusion that gripped those caught up in the events.
Andy Holey, who had gone to the arena to pick up his wife and daughter who had been at the concert, said: “An explosion went off and it threw me about 30ft from one set of doors to the other set of doors.
“When I got up I saw bodies lying on the ground. My first thought was to go into the arena to try to find my family.
“I managed to find them eventually and they’re OK.
“It was definitely an explosion and it was some force. It happened near the box office at the entrance to the arena.”
Media captionPeople can be seen running through Manchester Victoria Station
Emma Johnson said she and her husband were at the arena to pick up her children, aged 15 and 17.
“It was definitely a bomb. It was definitely in the foyer,” she told BBC Radio Manchester.
“We were stood at the top of the stairs and the glass exploded – it was near to where they were selling the merchandise.
“The whole building shook. There was a blast and then a flash of fire afterwards. There were bodies everywhere.”
BBC reporter Tom Mullen, who was at scene shortly after the blast, witnessed “sheer panic” among many young people, some with parents or guardians, in the city centre.
In the streets around the arena he saw concertgoers streaming away from the venue in confusion, many of them in tears.
Confusion and chaos after concert blast
Within an hour of reports of the incident emerging, people began offering spare rooms and beds to people stranded in the city using the hashtag #RoomForManchester.
Hundreds of tweets offering places to stay have been shared and re-tweeted thousands of times.
Other social media users began using the hashtag #MissingInManchester in an attempt to reunite people.
Majik Khan, 22, described the almost instantaneous stampede. “A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena,” Khan, 22, told Britain’s Press Association. “It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running toward us as they were trying to exit.”
Added Oliver Jones, 17: “The bang echoed around the foyer of the arena and people started to run.”
At least 19 people were killed and 50 others injured in the incident that police said Monday night is believed to be the work of terrorists.
Concertgoer Joe McElhone told CNN his bags were “definitely checked” before entering the arena, although he was not sure whether metal detectors were in use.
Outside the arena, medics tended to the injured, some whose clothes were shredded. There were hugs, tears and stunned looks. Many desperate parents and friends took to social media to search for loved ones they couldn’t find.
Dawn Price told The Bolton News, a northwestern England newspaper that’s part of the USA TODAY NETWORK, that she was attending the concert with her daughter and their friends. “I’m really shaken and can’t process what happened as so many people were there with children,” she said. “There was a big bang just as it finished and everyone started to run back into the arena. Then seconds later people started to run back in from another exit.”
Gary Walker of Leeds described a grim scene to BBC. Leeds said he and his wife could hear Grande’s closing song as they waited for their daughters to exit.
“And then suddenly there was a massive flash and then a bang and smoke. I felt a pain in my foot and my leg,” Leeds said. “I turned around to my wife … and she said, ‘I need to lay down.’ She’s got a stomach wound and possibly a broken leg. I’ve got a bit of a hole in my foot where I’ve got a bit of shrapnel.
“I was surprised I got away so lightly,” he said.
Manchester Arena, which opened in 1995, says on its website that it is one of the busiest venues in the world and the largest indoor arena in Europe. It has hosted such big names in entertainment as U2, Madonna and the Rolling Stones, as well as some high-profile sporting events.
“Our thoughts and prayers go out to the victims,” the arena said in a statement on the site Monday night.
Contributing: The Associated Press. Follow Susan Miller on Twitter: @susmiller